Formation of the modern Mekong subaqueous delta: a 3D view from Chirp sonar surveys

The Mekong River has formed one of the largest deltas in the south Asia, with a delta plain, which is approximately 50,000 km². Numerous sediment cores recovered from the Mekong Delta plain reveal that the delta has begun to form since the middle Holocene sea level highstand at around 6,000 a BP. Since then, it has prograded at least 250 km from the Cambodian border to the southeast Vietnamese coast. C-14 dating from sediment cores and OSL ages of the beach ridges indicate that the lower Mekong delta and the modern shorelines have been formed mainly in the past 3500 years, corresponding to the fall of late Holocene sea level. The Mekong’s subaqueous deltaic deposits represent the most recent seaward progradation and accumulation in the sea, and it has just been formed since no more than 1000 years ago.

To understand the distribution, thickness, sequence stratigraphy and sediment transport process, extensive chirp sonar surveys have been done immediately from the shore to the middle shelf, and along the shore from Vang Tau in the northeast to the tip of Ca Mau in the southwest. The 3-D high resolution chirp sonar profiles indicate clearly the distribution and thickness of the most-recent late Holocene deltaic deposits. Across the shore from its distributary channels to the shelf, the Mekong River has formed a classic clinoform with topset, foreset and bottomset, but just within water depth of -15m isobaths. Beyond this depth, the shelf is mainly dominated by relict sand. Along the shore, the Mekong derived sediment has been drifted more than 200 km southwestward to the tip of the Ca Mau peninsula and farther extending into the gulf of Thailand. The sediments near its distributary mouths are much coarser, consist of well-sorted fine sand, but become gradually finer along the shore--mainly fine mud around Ca Mau. The clay mineral, geochemical and sediment dynamics analysis over the subaqueous deltaic sediment have verified its Mekong’s origin.

The C-14 and Pb-210 data based on sediment cores obtained from the subaqueous delta have revealed the clinoform’s accumulation rates and patterns, see more in DeMaster’s abstract and presentation.